defined

Returns a Boolean value telling whether EXPR has a value other than the
undefined value undef. If EXPR is not present,
$_ is checked.

Many operations return undef to indicate failure, end
of file, system error, uninitialized variable, and other exceptional
conditions. This function allows you to distinguish
undef from other values. (A simple Boolean test will
not distinguish among undef, zero, the empty string,
and "0"
, which are all equally false.) Note that since
undef is a valid scalar, its presence doesn't
necessarily indicate an exceptional condition: pop
returns undef when its argument is an empty array,
or when the element to return happens to be undef.

You may also use defined(&func) to check whether subroutine func
has ever been defined. The return value is unaffected by any forward
declarations of func
. A subroutine that is not defined
may still be callable: its package may have an AUTOLOAD method that
makes it spring into existence the first time that it is called; see
perlsub.

Use of defined on aggregates (hashes and arrays) is
no longer supported. It used to report whether memory for that
aggregate had ever been allocated. You should instead use a simple
test for size:

Note: Many folks tend to overuse defined and are
then surprised to discover that the number 0
and ""
(the
zero-length string) are, in fact, defined values. For example, if you
say

"ab" =~ /a(.*)b/;

The pattern match succeeds and $1
is defined, although it
matched "nothing". It didn't really fail to match anything. Rather, it
matched something that happened to be zero characters long. This is all
very above-board and honest. When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer. So you
should use defined only when questioning the
integrity of what you're trying to do. At other times, a simple
comparison to 0
or ""
is what you want.